Simple coupled PDE example

Hi there,

as an introduction to more complex scenarios i tried to setup a very simple coupled pde with the coefficient form. I chose a stationary 1D Darcian flow over a length of 2m like this

1. equation: rho*(dv/dx) = 0 (v - velocity, rho - density)
2. equation: v = -(k/mu)*(dp/dx) (p - pressure, k - permeability, mu - dynamic viscosity)

BC: p(x=0) = 20[MPa] and p(x=2) = 0[MPa]

I set u = {v,p} and the coefficients beta_11 = rho, beta_22 = k/mu and a_21 = 1 in combination with two Dirichlet BC's for the end nodes to fix p. Unfortunately it doesn't work . It's a well posed problem with a very simple solution which is why i believe i set up something wrong. I attached the corresponding comsol file.

Any help is much appreciated

best regards Roman Rahner
(phd student of Georg Dresen)